Recently, the miniature projection device usually uses three kinds of LED chips such as a red-light LED chip, a green-light LED chip and a blue-light LED chip. The red-light LED chip, the green-light LED chip and the blue-light LED chip may respectively emit the red light, the green light and the blue light such that the miniature projection device may project the full-color image. Since the blue-light LED and the violet-light LED have high light-emitting efficiency, they are used to provide a green light by exciting a phosphor, and the brightness of the green-light emitted therefrom is greater than that of the green-light emitted from the green-light LED chip under the same current condition.
Furthermore, the miniature projection device always has a lens made of the plastic in order to reduce the cost and the weight, but the lens made of the plastic may be easily aged and deteriorated when the blue light or the violet light having the over-high energy directly emits to the lens made of the plastic. In addition, the foregoing disadvantages may cause the reliability of the miniature projection device lower under a long term usage.